Modulation



Sept. 16, 1941.

R. HOFER MODULATION Filed May 7, 1938 5 Sheets-Sheet 2 v INVENTOR ,Q1/00M H0 En BY 7%@ 1 ATTORNEY Sept. 16, 1941. R. HOFER 2,256,202

MODULATION Filed May 7', 1958 5 SheetS-She-et 4 CARR/ER SUPPRESSED l g4 VE CARR/Ek WAVE v v I v v w 1 .J/DE -BA/YD NERGY INVENTOR R000 F 0F51? BY furl/4f ATTORNEY Sept. 16, 1941. R HOFER 2,256,202

` MODULATION Filed May 7, 1958 5 Sheets-Sheet 5 g L CARR/Ek l g`, WAVE C! 4 PHASE vH/Frm Ely n10 j -H/a slof-BAND fumer INVENTOR RUDUZF H ER BY )fg ATTORNEY Patented Sept. 16, 1941 UNITED STATES PATENT OFFICE MODULATION Rudolf Hofer, Berlin, Germany, assigner to Telefunken Gesellschaft fr Drahtlose Telegraphie m. b. H., Berln, Germany, a corporation of Germany Application May 7, 1938, Serial No. 206,578 In Germany May 20, 1937 14 Claims.

This application concerns a method of land a circuit for the modulation of high frequency waves with improved efliciency.

Various methods are known by which an economical consumption of power of high frequency and in the rst place the weight of the required I modulation transformer is too high. Furthermore, the efficiency of thevtransmitter can be improved by the modulation method of Chireix,

-but the eiciency of this method decreases rapidly with an increase in the degree of modulation, and its control of operation is extremely diliicult. In another modulation method also already Vknown the transmitter comprises two stages operating in parallel into a common load and of which one stage transmits only these high frequency amplitudes lying above the mean carrier value while the other one transmits only the modulation amplitudes lying below said carrier value. In this case, the linearity of the dynamic modulation characteristic is again questionable in case of small degrees of modulation as Well as at 1 very high degrees of modulation so that cumbersome arrangements for reducing the distortion for the purpose of improving the quality are necessary.

The object of the invention is to provide a circuit for modulation with improved efficiency whereby the latter is more favorable than in case of the last mentioned method and in which the modulation characteristic is at the same time practically completely straight. In accordance with the invention, the carrier energy of a nonmodulated tube stage is delivered to the load and the additional modulation energy is added by means of a stage consisting of one or several tubes connected in series or in parallel to the load, said stage furnishing the load with high frequency energy during the half cycles of the modulation during which the wave amplitude exceeds the carrier amplitude but absorbing high frequency energy from the load during the other half cycles of modulation.

In order to practice this method various modes of construction are possible, two of which will be described in greater detail in the following. The essential point in the two examples of construction resides in that the efficiency of the tube supplying the carrier energy is rendered an optimum, for instance in that the tube operates approximately between a condition of low efliciency with high output and a condition of very high` efficiency but low output. Care must then simply be taken that the eiciency of the stage furnishing the modulation power has the same high value or permits at least of attaining a high total efficiency.

In describing my invention reference will be made to the attached drawings wherein:

Fig. 1 is a circuit diagram illustrating a modulation system, arranged in accordance with my invention, and comprising a non-modulated carrier amplifier stage coupled to a load with an additional modulator stage for supplying energy to the load and to the output of the amplifier stage when the modulated wave under control of modulating potentials exceeds the carrier amplitude and for absorbing energy from the load when the modulated wave amplitude is less than carrier amplitude.

Figs. 2a, 2b, 3, 3a and 4 are graphs and curves used in describing operation of the system illustrated in Fig. 1.

Figs. 5, 6, 7, 8 and 9 illustrate modications of the arrangement of Fig. 1, while;

Fig. 10 is a curve illustrating the operation of the modulator in the system.

In Figure l the tube operating with optimum eniciency and serving for providing the carrier energy is designated by R1. This tube is so dimensioned as to be capable of furnishing at least twice the average carrier energy. The grid of said tube has the high frequency control potential Uyl applied thereto. The plate circuit contains the tuned oscillatory circuit L1, C1 through which the upper harmonics are passed to ground.

tended into tuned series circuits by means of capacities CK2 and CK3 respectively. In the case of all tubes, blocking condensers CB are inserted between the anodes and the oscillatory circuits,Y

Y half cycle in which R3 is conductive.

additional power'during the positive/half cycles Y,

of modulation,` thatV is, when the power supplied to the load is above the normal carrier power,r while the tube R3 produces across the coupling LKs, L1 a counterpotential in theV load circuit Y during the negative half cycles of the modulation,v

so that the potential at load R will be decreased. Thus, the tube R3 consumes only the energy which itqreceives from the tube'R1 therefore not requiring in general any direct plate potential.

Only where very high degrees of modulation appear such as up to 100% will a relatively low direct potential Us be of advantage.

. I Awill now describe the operation of the tube VR2 in the upward or positive half'cycle of modulation. YThe negative grid biasing potential Uy blocks the tube almost completely, i. e., the peak of the high frequency potential Uyz impressed upon the grid of the tube causes practically no flow of plate current. These conditions are shown in Figure 2a. The working point A of the tube R2 thus is located in the negative region at the voltage Uy, so that the high frequency amplitude Ugg just reaches up to the zero point of the grid potential Uy. The plate current is designated by ia and the characteristic of the tube 4by S. On the positive half cycle of modulation a voltage Uma is applied to the grid of tube R2 whereby the working point is shifted in the positive direction. The modulation potential Umz is indicated in dash lines in Figure 2a. During a positive half cycle of modulation, the tube R2 passes current and thereby applies across the coupling LK2, L'1 a voltage to the load R, said voltage doubling the potential at R in case of 100% modulation. Therefore, the current in the load circuit passing through the load R and tube R1 will likewise be doubled. The phase'of the voltage applied from tube R2 to the load R must correspond kwith the phase of the voltage applied from tube R1 to the load. Since in the circuit shown a phase shift of occurs inthe coupling circuits between the .l

alternating-current potential at R and the alternating plate potential of tube R2, the exciting voltage Ugg applied to the grid side of the tube R2 must likewise be shifted in phase by 90 with respect to the exciting voltage Ug1 of tube R1.

This phase relation is obtained Vin any manner such-as, for example,iby a phase shifter between tbe Vcarrier source and points 2 and 4. When assuming that the direct current plate potential of tubes R1 and Rz are equal to each other, such as is generally. the-case Vwhen usinga common plate potential source, the coupling LK2, L1 must Ytube R3 will become conducting because the modulation potentials are. in opposite phase ony the grids of tubesrRz and R3. R2 is now blocked. The

be adjusted in suchr manner that the voltage conditions at tube Ra are represented in Figure 2b. The biasing grid potential of R3 is preferably chosen equal to that of tube R2, i. e., equal to Uy suitablyV by using the same voltage source. The alternating grid potential of high frequency is designated by U93, and the alternating potential Yof low frequency is designated by Uma.

Since the phase of the high frequency potential applied from the tube R3 to the load is in opposition to the phase of the high frequency potential obtained from the tube R1, the tube R3 consumes the power received from the tube R1 during the words on this part of the modulation cycle R1 supplies power to R3 and this said power is` converted by said tube R3 into anode heating losses. This energy produces'across the coupling LKs, L1 in the load circuit a counterpotential by which the load power will be reduced. The current passing through the tube R1 is diminished thereby and will be equal tozero at modulation, while the alternating plate potential at R1 remains practically constant..

Figures 3 and 3a show-the conditions of the voltages and currents of tubes R2 and R3 during an entire modulation cycle (in this case as well as inthe remaining part of the description it is assumed that the transmitter is modulated only Ywith a single'audio-frequency oscillation; if the modulation consists of a vfrequency mixture, for instance speech or music, obviously also this mixture can always be divided into parts having positive modulation and parts having negative modulation). The tube R2 (FigureV 3) operatesI as regards high frequency in equal phase with theV voltage suppliedby the tube R1. The maxima -lof the currentl of tube R2 (shaded areas) correspond with the voltage minima in the half cycles of positive modulation. V.The direct plate potential is designated by Ua, the alternating plate potential by-Uaz and the plate current-by iaz.

AInrthe half cycle in4 which the modulation is ycurrent. maxima correspond in this latter tube with the voltage maxima. The plate'current is designated by ias, the direct plate potentialV by UZ, the alternating plate potential by Uaa. The voltage UZ is at a high degree of modulation necessary only in view of the fact that the altermating plate potential becomes very small or even zero just at the moment in which the tube .is passed by the maximum plate current. In this moment a minimum plate potential must exist at the tube and which is approximately equal to the saturation voltage.

The modulation characteristic of the entire circuit depends upon the characteristics of the two tubes R2 and Ra. The total characteristic can be madelinear by slightly opening the tubes R2 and R3 at the gridside in the' non-modulated state such as shown in Figure 4. This is done by reducing the bias Ug slightly. In this gure S2 represents the characteristic of the tube R2 while Ss is that of the tube R3. Item T represents the average carrier value, i. e., the value corresponding to the non-modulated state. Then even without modulation tube Rz supplies a certain ,currentamplitude Iao and a certain power which will be consumed by the tube R3 and is converted into heat. Since the lower bends of the modulation In other I characteristics of R2 and R3 can practically be rendered equal to each other through voltage feed back or countercoupling there will be obtained in a manner similar to the push-pull class B modulation, a resultant straight modulation characteristic if the additional carrier current Iao is properly adjusted.

As regards the value of the necessary power of the tubes, the following can be said: At a modulation of 100% each of the two tubes R1 and R2 must be capable of delivering as peak value a power equal to twice the power of the carrier. If in the non-modulated state equal voltages would `be furnished from R1 and transmitted to R2 and R3 (voltages of the order of one-half of the direct voltage Ua) then R3 would, the same as R2 at the moment of the voltage doubling, have to furnish the same maximum current in order to suppress this voltage. The voltage peaks at the tubes R1 and R2 have a value equal to twice that of the direct potential. Therefore, when admitting the same value of the voltage peak also for tube R3, the maximum voltage which the tube R1 delivers to the tube R3 can approximately be four times as high as the voltage supplied by R1 to R2, because hardly any direct potential exists at R3. This voltage transmission can be set by means of the coupling LKa, L1. In this case the tube R3 need have only one-half of the emission of R2, hence also only one-half of the actual power as related to the same plate potential. The actual power may even be still lower since voltage peaks occur always in the non-modulated state, so that a higher potential can be applied to the tube. This condition as regards power indicates that the required additional number of tubes is unimportant as regards other modulation circuits.

When utilizing a coupling coil LK2 and LIQ which is in part a common 'coupling coil, the following condition should be borne in mind: As long as the tube R1 only furnishes current it provides the tubes R2 and R3 with voltages across these coupling coils, which voltages are displaced in phase by 90 relative to the control power of Ug1 of R1. Now, if one of the two tubes R2 or R3 provides an additional voltage, the said tube causes at the anode of the other tube (R3 or R2) a partial voltage which is displaced in phase by 90 relative to the voltage part supplied by R1. As long as only one of the two tubes R2 or R3 is in operation, this phase displacement is immaterial. During the short time periods of the common operation, the phase displacement, however, has the effect in the two tubes as if said tubes would operate upon a re'- sistor having a small wattless component instead of operating upon an effective resistance. Since, however, during the short time periods of the common operation of the two tubes, the output of power is extremely small as compared with that of the tube R1, this condition can practically be neglected.

In the circuit shown in Figure l the stage furnishing the additional modulation power (and comprising the tubes R2 and R3) is placed in series to the load R. In place of this series connection also a parallel connection may be used in which case special means are provided, however, whereby the tubes operate in common in the desired manner. Such means have in part become already known in the modulation circuit mentioned above in which two stages operating in parallel act upon a common load. The following explanations, therefore, are brief. In

Figure 5 the tubes R2 and R3 which are controlled in the same manner as in Figure 1, operate upon a common blocking circuit L2 C2 and upon the load R, i. e., they have the same alternating plate potential applied thereto. Hence, the emission of R3 must 'be equal to that of R2. Between the tube R1 furnishing the carrier power, and the load, a transformation member is inserted consisting of either a known )1/4 connection, or as represented in the drawings, of a substitution circuit which is an equivalent of this connection and formed by the inductance L1 and capacities C1 and CK2. The other reference characters of Figure 5 correspond to those of Figure 1. The carrier tube R1 operates at optimum efficiency which is accomplished by the combination of the capacity CB and of the parallel resistor Rg. Similar to the transformation member LK2 and LKa in Figure 1, the parallel circuit may also be closed across a transformation member situated between the tube R1 and the tubes R2 and R3 as shown in Figure 6, which in view of the descriptions of the other figures does not require further elucidation. In this last mentioned circuit a tube having diminished power may again be utilized as tube R3. Finally, it is also possible to provide a series connection in accordance with Figure 1 and having no transformation member LKz, LK3 as was the case in the parallel connection of Figure 5, whereby the powers of the tubes R2 and R3 must again correspond to each other. The tube R1 which may be as in the prior figures supplies carrier power to L1. This may be done by connecting the output of R1 to points I'and IB of Figure 6.

In a second example of construction which will be described in connection with Figures '7-10, the carrier power is again furnished by a non-modulated tube having a high efficiency, while the stage serving for furnishing the modu- .lation power consists of an amplifier controlled by the modulated high frequency with suppressed carrier. Again, a series connection as well as a parallel connection of this amplifier stage with the load is possible. The series connection is shown in Figure 7 in which R1 designates the tube furnishing the carrier power and R is the load. As in the aforegoing example of construction items L1, C1 represent again a tuned series circuit and L1, C2 represent a blocking circuit. The grid biasing potential of tube R1 adjusts itself automatically to the optimum efficiency of the tube on account o-f the combina tion of-Rg and CB. The emission of tube R1 must again be so high that a control up to the double value of the carrier power is possible. 'I'he tube R2 has such a grid biasing potential (Uy) applied thereto that it operates with a small steady plate current which is such that the oscillation curve of the tube is practically a straight line. As alternating grid potential Ugz of tube R2 for instance the output voltage of a push-pull modulator may be used in which the high frequency is modulated by the modulation potentials but the carrier is suppressed. Therefore, in the non-modulated state the tube R2 does not receive alternating grid potential. The plate circuit of tube R2 contains the tuned oscillatory circuit L2, C2 coupled -to the inductance L1 across an intermediate circuit LKi, LK2. If the tube R1 supplies carrier power a voltage will be applied to the anode of tube R2 across this coupling, said voltage being so chosen by suitable dimensioning of the coupling that it is approximately equal to one-half of the direct plate l.plate volta-ge source.

l ciency of this tube is constantly favorable.

potential Ua. This latter condition is obviously lonly true if the direct plate potential of tube R1 and' that of `tube R2 are the same such as willv generally be the case when using a common The tube R2 must have a high internal resistance in order that the voltage impressed upon the anode of R2 by the tube R1 causes a substantial plate current thus avoiding any considerable losses. Since the internal resistance of the tube R1 is very low due to the limited operation between the state of operation with excessive voltage and that of insuicient voltage, practically. the entire side band power` is applied fromthe tube R2 to the load R. This side band power prod ces -at the circuit L'1, C1 a resultant voltage whose phase position must correspond with the phase position oi the carrier. Since, however, a phase change of 90 occurs in the coupling circuits L2, LK1,YL&, L1

the said required condition can beattained onlyv if the phase ofthe alternating grid potental UQ@l is dispiaced by 90 relative ,to the alternating grid potential Uyl.

nected as shown dagrammatically in Figure 1.`

In the following the Voperation of the two tubes R1 and R2 at modulation will be elucidated: The

tube AR1 operates with an alternating plate po-` tential which remainsY practically constant and whose amplitude has approximately the same value asthe direct plate potential. Since, the side band currents furnished by R2 must flow through the tube R1, the alternating plate current of R1 pulsates in the rhythm of the modulation without the alternating potential of tube R1 being influenced thereby. Therefore, the efli- Figure 10 shows the variations of the plate current ia (shaded areas) and the plate potential of tube R2 at 100% modulation with a sinusoidalfv note during an audio-frequency cycle. The alternating potential transmitted from tube R1 to the anode of tube R2 is approximately one-half of the direct plate potential. In thegure the I, voltages and the currents in the half' cycle forf upward control, i. e., at an increase in power, are shown between and 1r. The carrier voltage Uli/2 is controlled upwards by Ua whereby Y thecurrent attains its maximum value at the time of the lowest steady plate potential duringr Throughout the high frequency half cycles. this half cycle the efciency of the tube R2 is favorable because the plate current pulses occur when plate current is minimum. During the sec-` supply side-band energy only to Rz. The plate circuit L2, C2 of this tube operates in parallel to the load YR placed in series to the coil L1 this operation taking place across a coupling coil LKi tuned in conjunction with the coil LKz and the capa ity CK. This `circuit corresponds with the circuit according to Figures 5 and 6. Figure 9 nally shows an arrangement in which the tube R2 operates likewise in parallel upon the load, whereby however the tube R1 is connected to the load across an energy line whose length is M4. In both circuits (Figure 8 and Figure 9) the current passing through the tube R1 varies in the rhythm of the modulation. .These current variations are caused by the change of the apparent damping of the load circuit.

In the arrangements of Figures 7-9 the modulation power is furnished by an amplifier tube controlled by the modulated high frequency having'the carrier suppressed. However, the tube R2 may also be substituted bythe modulator for suppressing the carrier.` If this modulator is a push-pull modulator there can be seen immediately the similarity between the circuit (Figure l) described in connection with the rst example embodying the idea of the invention and the second example. The sole difference resides in that in Figure 1 the one push-pull tube R3 receives no direct plate potential or only a low potential, while the'voltage conditions in the push-pull modulator are symmetrical. This distinction entails an essential difference between Vthe modes of operation of the two circuits.

The invention is in no ways limited to the special circuits shown herein and may be modied in many ways, for instance in that amplier stages are inserted between the individual tubes, or in that in each stage several tubes operate in parallel for the purpose of increasing the power, etc.

I claim:

1. In a high eiciency carrier wave modulating and amplifying system, a load circuit, a source of carrier waves, an amplier having an input coupled to said carrier wave source and having output electrodes coupled to said load circuit, a

'carrier suppressor modulator comprising a pair of tubes having input .electrodes coupled to said carrier wave source and having output electrodes coupled by separate circuits to said load circuit' and to the output electrodes of said amplifier, means for controlling the amplification of the tubes of said modulator in push-pull relation in accordance withV signal potentials, and means for producing a phase relation between the carlated state the total eiliciency of the arrange-V ment still lies approximately at Gil-%.

Figure V8 vshows a circuit in which the tube R2 of Figure 7 is placed in parallel to the load R and not in series thereto as in Figurev 7 In Figure 8'for simplicity the carrier suppressed modulator has beenomitted. It may be as in Figure 'l and may be connected at points 2li and 22 to riers supplied to the inputs of said amplifier and l modulator such that the energy supplied by said modulator to said load and to the output electrodes of said amplifier is in phase with the carrier energy on the output electrodes of said amplier during the positive half cycles of the modulating potentials and is in phase opposition during the negative half cycles of said modulating potentials.

'2. The method of modulation which comprises the steps of, producingcarrier wave energy, am-

plifying said carrier wave energy in a rst path,Y

modulating said-carrier wave energy in a second path at signal frequency to produce sideband energy the phase ofv which reverses with reversals in the modulation cycle, relatively adjusting the phases of the carriers so that the phase of theV plied carrier wave energy, and combining said energies to augment on one-half of the modulation cycle and oppose on the other half of the modulation cycle, and to control the amplification of said carrier energy by said sideband energy.

3. The method of modulation which comprises the steps of, producing carrier wave energy, amplifying the said carrier wave energy in a first path, modulating said carrier wave energy in a second path at signal frequency to produce modulation energy comprising sideband energy the phase of which reverses with reversals in the signal frequency cycle, adjusting the relative phases of the carriers to a Value such that the said sideband energy is in phase with the said amplified carrier during a portion of the sideband cycle, and imposing said modulation energy on said amplified carrier energy to control the amplification of the amplified carrier wave energy and simultaneously to augment said carrier on onehalf of the modulation cycle and oppose said carrier on the other half of said modulation cycle.

4. In a carrier wave modulating system, a load circuit, a source of carrier waves, an ampliiier tube having input electrodes coupled to said carrier wave source and having output electrodes coupled to said load circuit, a carrier suppressor modulator comprising a pair of tubes having input electrodes coupled to said carrier wave source and having output electrodes coupled to said load circuit and to the output electrodes of said amplifier, means including a source of modulating potentials for controlling the amplification of the pair of tubes of said modulator in push-pull relation in accordance with signal potentials, means for biasing the tubes of said pair of tubes by potentials such that their outputs are low in the absence of signal potentials, and means for producing a phase relation between the carrier waves supplied to the input of said amplifier and the inputs of said modulator tubes such that the energy supplied by said modulator tubes to said load circuit and to the output electrodes of said amplier tube is in phase with the carrier wave energy supplied by said amplifier to said load circuit during the positive half cycles of the signal potentials and is in phase opposition during the negative half cycles of the signal potentials.

5. A system as recited in claim 4 wherein the output electrodes of said amplifier are coupled by a series tuned circuit with said load circuit and wherein the output electrodes of said pair of modulator tubes are coupled with said series tuned circuit to combine said outputs in series in said load circuit.

6. A system as recited in claim 4 wherein the output electrodes of said ampliiier tube are connected in a series circuit tuned to the frequency of said carrier wave energy and connected in series with said load circuit and wherein the output electrodes of said pair of tubes of said modulator are connected in parallel and coupled to said circuit series-tuned to the carrier wave frequency.

7. In a wave modulating system, a load circuit, a source of carrier wave energy, an amplifier having input electrodes coupled to said carrier wave source and having output electrodes connected in series to said load, a carrier suppressor modulator arrangement having input electrodes and output electrodes, phase shifting means coupling said input electrodes to said carrier wave source, means coupling said output electrodes to said load and to the output electrodes of said ampliiier tube, said coupling being such that the output of said modulator is in series with the output of said amplifier, means for controlling the amplification of said modulator tube arrangement in accordance with signals whereby wave energy modulated in accordance with modulating potentials is produced therein, and means for biassing said modulating tube system by a potential such that it supplies substantially no energy to said load and the output electrodes of said amplifier in the absence of modulating potential signals.

8. In a carrier wave modulating system, a source of carrier wave energy to be modulated, a source of modulating potentials, a load irnpedance, a carrier wave amplier tube having input and output electrodes, means coupling said input electrodes to said source of wave energy, a circuit coupling the output electrodes of said amplifier tube to said load impedance, a pair of modulator tubes each having input and output electrodes, means for impressing wave energy from said source in phase opposition on the input electrodes of said modulator tubes, means for coupling the output electrodes of said modulator tubes in parallel and impressing their output on said load impedance and on the output electrodes of said amplifier tube, means for modulating the impedances of said modulator tubes in phase opposition whereby modulation of said carrier is accomplished with carrier suppression, means for biassing said modulator tubes by a direct-current potential such that their output is substantially zero in the absence of modulating potentials, and means for relatively adjusting the phases of the carrier energy impressed on the input electrodes of said amplier tube and the carrier energy impressed on the input electrodes of said modulator tubes to a Value such that the output of said modulator tubes is in phase with the output of said amplier tube on the positive portions of the modulation cycle.

9. A system as recited in claim 8 wherein said phase shifting means is included in said means for impressing wave energy from said source on the input electrodes of said modulator tubes.

10. A system as recited in claim 8 wherein said phase shifting means is in said circuit coupling the output electrodes of said amplifier tube to said load impedance.

1l. A system as recited in claim 4 wherein said last named means is a line the length of which is substantially equal to one-quarter the length of the carrier waves.

12. A system as recited in claim 8 wherein the output electrodes of said amplifier tube are coupled in series with the load circuit by a circuit series-tuned to the frequency of the carrier waves, and the output electrodes of said pair of modulator tubes are coupled to said series-tuned circuit.

13. A system as recited in claim 4 wherein said coupling between the output electrodes of said modulator tubes and said load circuit and the output electrodes of said amplifier tube includes a coupling tube coupled at its input to said output electrodes of said modulator tubes and at its output to said load circuit and the output electrode of said amplifier tube.

14. In a wave modulating system, a source of carrier wave energy voltage to be modulated, a source of modulating potentials, a load circuit, three electron discharge tube systems each having input and output electrodes, one of said tube systems operating as an amplifier, means coupling said source of carrier wave energy to the Vcite the same in phase opposition by modulating input electrodes ot' all of saticl-V` tube systems to potentials to produ-ee. therein: voltages of.' upper' excite. the same cci-linearlyr by wave energy Voltemd'lowex sideband frequencies, and.. means eeuages, said one of said. tube systemsvoperating to pliing the output eleetrodes ot' all of said tubeA amplify said Wave energy, means coupling .said systems: te setidloabd circuit to. combine sai amsource of modulating potentials to the input elec- 5 plied carrier Wave Voltage and said si'deband trede of the other two of said tube systems to exvoltages in series in load circuit.

Y Y RUDOLF 

